我有一个包含以下表格和字段的MySQL数据库：
学生和班级表在id（主键）上编制索引。成绩表在id（主键）和student_id，class_id和grade。
上编制索引我需要构建一个查询，给定一个类ID，给出所有其他类的列表以及在其他类中获得更多分数的学生数。
基本上，鉴于成绩表中的以下数据：
student_id | class_id | grade
--------------------------------------
1 | 1 | 87
1 | 2 | 91
1 | 3 | 75
2 | 1 | 68
2 | 2 | 95
2 | 3 | 84
3 | 1 | 76
3 | 2 | 88
3 | 3 | 71
使用类ID 1查询应该产生：
class_id | total
-------------------
2 | 3
3 | 1
理想情况下，我希望在几秒钟内执行此操作，因为我希望它能够成为Web界面的一部分。
我遇到的问题是，在我的数据库中，我有超过1300个班级和160,000名学生。我的成绩表有近1500万行，因此查询需要很长时间才能执行。
这是我到目前为止所尝试的以及每次查询所花费的时间：
-- I manually stopped execution after 2 hours
SELECT c.id, COUNT(*) AS total
FROM classes c
INNER JOIN grades a ON a.class_id = c.id
INNER JOIN grades b ON b.grade < a.grade AND
a.student_id = b.student_id AND
b.class_id = 1
WHERE c.id != 1 AND
GROUP BY c.id
-- I manually stopped execution after 20 minutes
SELECT c.id,
(
SELECT COUNT(*)
FROM grades g
WHERE g.class_id = c.id AND g.grade > (
SELECT grade
FROM grades
WHERE student_id = g.student_id AND
class_id = 1
)
) AS total
FROM classes c
WHERE c.id != 1;
-- 1 min 12 sec
CREATE TEMPORARY TABLE temp_blah (student_id INT(11) PRIMARY KEY, grade INT);
INSERT INTO temp_blah SELECT student_id, grade FROM grades WHERE class_id = 1;
SELECT o.id,
(
SELECT COUNT(*)
FROM grades g
INNER JOIN temp_blah t ON g.student_id = t.student_id
WHERE g.class_id = c.id AND t.grade < g.grade
) AS total
FROM classes c
WHERE c.id != 1;
-- Same thing but with joins instead of a subquery - 1 min 54 sec
SELECT c.id,
COUNT(*) AS total
FROM classes c
INNER JOIN grades g ON c.id = p.class_id
INNER JOIN temp_blah t ON g.student_id = t.student_id
WHERE c.id != 1
GROUP BY c.id;
我还考虑创建一个2D表，学生作为行和类作为列，但是我可以看到两个问题：
我还考虑将这些计算作为后台作业并将结果存储在某处，但为了使信息保持最新（必须），每次创建或更新学生，班级或成绩记录时都需要重新计算。
有没有人知道构建此查询的更有效方法？
编辑：创建表语句：
CREATE TABLE `classes` (
`id` int(11) NOT NULL AUTO_INCREMENT,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=1331 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci$$
CREATE TABLE `students` (
`id` int(11) NOT NULL AUTO_INCREMENT,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=160803 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci$$
CREATE TABLE `grades` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`student_id` int(11) DEFAULT NULL,
`class_id` int(11) DEFAULT NULL,
`grade` int(11) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `index_grades_on_student_id` (`student_id`),
KEY `index_grades_on_class_id` (`class_id`),
KEY `index_grades_on_grade` (`grade`)
) ENGINE=InnoDB AUTO_INCREMENT=15507698 DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci$$
关于最有效查询的解释输出（1分12秒一）：
id | select_type | table | type | possible_keys | key | key_len | ref | rows | extra
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
1 | PRIMARY | c | range | PRIMARY | PRIMARY | 4 | | 683 | Using where; Using index
2 | DEPENDENT SUBQUERY | g | ref | index_grades_on_student_id,index_grades_on_class_id,index_grades_on_grade | index_grades_on_class_id | 5 | mydb.c.id | 830393 | Using where
2 | DEPENDENT SUBQUERY | t | eq_ref | PRIMARY | PRIMARY | 4 | mydb.g.student_id | 1 | Using where
另一个编辑 - 解释sgeddes建议的输出：
+----+-------------+------------+--------+---------------+------+---------+------+----------+----------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------+--------+---------------+------+---------+------+----------+----------------------------------------------+
| 1 | PRIMARY | <derived2> | ALL | NULL | NULL | NULL | NULL | 14953992 | Using where; Using temporary; Using filesort |
| 2 | DERIVED | <derived3> | system | NULL | NULL | NULL | NULL | 1 | Using filesort |
| 2 | DERIVED | G | ALL | NULL | NULL | NULL | NULL | 15115388 | |
| 3 | DERIVED | NULL | NULL | NULL | NULL | NULL | NULL | NULL | No tables used |
+----+-------------+------------+--------+---------------+------+---------+------+----------+----------------------------------------------+
答案 0 :(得分：3)
我认为这应该适用于SUM和CASE：
SELECT C.Id,
SUM(
CASE
WHEN G.Grade > C2.Grade THEN 1 ELSE 0
END
)
FROM Class C
INNER JOIN Grade G ON C.Id = G.Class_Id
LEFT JOIN (
SELECT Grade, Student_Id, Class_Id
FROM Class
JOIN Grade ON Class.Id = Grade.Class_Id
WHERE Class.Id = 1
) C2 ON G.Student_Id = C2.Student_Id
WHERE C.Id <> 1
GROUP BY C.Id
<强> - 编辑 -
在回应你的评论时，这是另一个应该更快的尝试：
SELECT
Class_Id,
SUM(CASE WHEN Grade > minGrade THEN 1 ELSE 0 END)
FROM
(
SELECT
Student_Id,
@classToCheck:=
IF(G.Class_Id = 1, Grade, @classToCheck) minGrade ,
Class_Id,
Grade
FROM Grade G
JOIN (SELECT @classToCheck:= 0) t
ORDER BY Student_Id, IF(Class_Id = 1, 0, 1)
) t
WHERE Class_Id <> 1
GROUP BY Class_ID
答案 1 :(得分：0)
您是否也可以尝试原始数据！它只有一个加入：）
select
final.class_id, count(*) as total
from
(
select * from
(select student_id as p_student_id, grade as p_grade from table1 where class_id = 1) as partial
inner join table1 on table1.student_id = partial.p_student_id
where table1.class_id <> 1 and table1.grade > partial.p_grade
) as final
group by
final.class_id;